In recent years, in order to perform power supply or perform charging, wireless power transfer techniques have been gaining attention. Research and development are being conducted regarding a wireless power transfer system wirelessly performing power transfer to various electronic apparatuses such as mobile terminals and notebook computers and household electrical appliances or to power infrastructure equipment.
When wireless power transfer is used, standardization is preferably performed so that power sources which transmit power and power receivers which receive the power transmitted from the power sources are used without trouble even when they are products manufactured by different manufacturers.
Conventionally, techniques using electromagnetic induction, and techniques using radio waves are generally known as wireless power transfer techniques.
Recently, wireless power transfer techniques using strong coupling resonance have been attracting attention as techniques being capable of transferring power to a plurality of power receivers while placing each power receiver at a certain distance from a power source, and to various three-dimensional postures of each power receiver.
Wireless power transfer techniques using magnetic field resonance or electric field resonance, for example, are known as this kind of wireless power transfer using strong coupling resonance.
Conventionally, in order to perform power supply or perform charging, wireless power transfer techniques for wirelessly transferring power have been gaining attention, as described earlier. A wireless power transfer system which employs such a wireless power transfer technique normally transfers power to a plurality of power receivers and may involve power transfer control based on, for example, the power required by each power receiver or the positional relationship of each power receiver relative to the power source.
Such power transfer from a power source to a plurality of power receivers includes time-division power transfer in which power is transferred for each power receiver, and simultaneous power transfer in which power is simultaneously transferred to a plurality of power receivers. However, a wireless power transfer system including a plurality of power receivers does not perform wireless power transfer based on an evaluation index set for each power receiver, and it is therefore difficult for this system to perform appropriate wireless power transfer to each power receiver.
A variety of wireless power transfer techniques have conventionally been proposed.    Patent Document 1: Japanese Laid-open Patent Publication No. 2012-044735    Patent Document 2: International Publication No. WO 2013/035873 pamphlet